The present invention relates to a composition and a method for priming and coating a moisture-cured silicone substrate, and to the coated substrate obtained therefrom. More specifically, the present invention relates to a method comprising treating the surface of a moisture-cured silicone substrate with a new primer composition and thereafter applying a coating composition to the primed surface to provide a silicone substrate having a durably adhered coating composition.
Silicone substrates comprising a cured silicone polymer typically have low surface energies and organic coatings do not wet the surface of the silicone substrate well. Consequently, a poorly or non-adhering overcoating results when one attempts to simply coat a silicone substrate with an organic material.
Several attempts to solve this problem have been disclosed in the art, all involving the mixing of an additional component, typically called an adhesion promoter or a primer or a coupling agent or a paintability component, into a curable silicone composition prior to its being applied and cured. For example, U.S. Pat. Nos. 3,542,714; 3,812,164; 4,593,065; and 3,957,714 recognize the paintability problem associated with a cured silicone surface and disclosed the addition of rosin, amidoalkylthioalkyl-substituted alkoxysilanes, aminoxy- and/or amido-substituted silicon compounds and calcium carbonate, respectively, to curable silicone compositions to improve the paintability of the composition once it has been cured. U.S. Pat. No. 3,817,894 discloses a silicone latex caulk comprising an aminofunctional silane which is said to function as a crosslinking agent and as a curing catalyst for the siloxane copolymer component of the composition. When cured the composition has good paintability with latex or oil-based paints.
This method of achieving paintability of a cured silicone surface has drawbacks. For example, one must mix the additional component into the silicone composition before the composition is cured. Consequently, this method is not available when one wishes to coat an already cured silicone substrate. Furthermore, the mixing of an additional component into a formulated silicone composition may alter the physical properties of the curable and/or cured composition in an undesirable way, while achieving the desired paintability.
Amino-functional silicon compounds have been used to improve the adhesion of various materials to a cured silicone coating; however, it appears that such a method has not been extended to the coating of a silicone substrate. For example, U.S. Pat. No. 4,315,970 claims a method for forming an adherent metal surface on a substrate by treating the substrate with a curable organic or silicone basecoat, curing said basecoat, treating the cured basecoat with at least one organofunctional silane or partial hydrolyzate thereof, drying the silane-treated surface and vapor-depositing a metal on the silane-treated surface. The organofunctional silane has the formula (RO).sub.a Si(R).sub.c R'X wherein --R'X can be, for example, --CH.sub.2 CH.sub.2 CH.sub.2 NHCH.sub.2 CH.sub.2 NH.sub.2, and the basecoat is preferably a silicone basecoat, and more preferably a basecoat selected from the siloxane coatings disclosed in U.S. Pat. No. 4,986,997. Also, U.S. Pat. No. 3,477,901 discloses a method for adhering a polyolefin to a substrate comprising treating the surface with a mixture of silanes, one of which is an amino-functional silane, drying the treated substrate and bonding the polyolefin to the dried substrate using heat and pressure. The substrate can be, for example, a silicone-treated parchment paper.
These methods of overcoating, comprising the use of vapor deposition, mechanical pressure or other complex means, are not suitable for the coating of silicone substrates, such as those one would likely find in the sealants and caulks of a building construction.
Amino-functional silicon compounds have been used extensively to improve the adhesion of a curable silicone composition to various substrates, such as glass, metal, fillers, etc. Mention is made of U.S. Pat. Nos. 3,554,952; 3,888,815 and 4,681,808, the latter disclosing a method for improving the adhesion of a silicone elastomeric coating to a polyurethane substrate comprising applying to the polyurethane substrate a limited amount of a silane having the formula ZSi(OR).sub.3, where Z is an amine-functional radical and R is a monovalent hydrocarbon radical having from 1-6 carbon atoms, and then applying a coating of a specific curable silicone coating to the thus-treated polyurethane substrate.
There is a need for a method for coating a silicone substrate, particularly a building sealant, with a durably adhering protective and/or decorative coating, which does not require the mixing of a component with the silicone prior to its being applied and cured, or the using of complex overcoating methods, such as vapor deposition or mechanical pressure.